A demagnifying electron-beam projection system may be used as a lithographic device for exposing integrated circuit patterns onto semiconductor wafers. In such systems, an electron beam irradiates a mask defining a desired pattern. To better control aberrations, this type of system does not simultaneously illuminate the entire mask (or even an entire die) for exposure; consequently, the pattern on the mask is divided into many tiny regions termed "mask subfields." The electron beam illuminates the mask subfields individually and sequentially. A reduced image of each illuminated subfield of the mask is formed via a two-stage projection lens on corresponding "transfer subfields" on a sensitized wafer or other substrate. (For an example of such a system, see Japanese Patent Publication No. Hei 5-160012, incorporated herein by reference. For a discussion of applications of this technique, see, for example, U.S. Pat. Nos. 5,260,151 and 5,466,904, incorporated herein by reference).
Typical lens systems and techniques used in demagnifying electron-beam projection lithography systems include: (1) MOL (Moving Objective Lens) and VAL (Variable Axis Lens) systems, in which magnetic fields formed by axis deflectors are applied to the magnetic field of the lens so as to shift the electron-optical axis of the lens, and the symmetric magnetic doublet lens system, which represents an attempt to suitably minimize certain aberrations in the electron-optical system.
With the symmetric magnetic doublet, all anisotropic (rotational) aberrations, chromatic magnification aberrations, and distortion are zero. (See, for example, M. B. Heritage, "Electron-Projection Microfabrication System," J. Vac. Sci. Technol., Vol. 12, No. 6, November. December. 1975, incorporated herein by reference). In the symmetric magnetic doublet lens system, however, field curvature cannot be completely corrected. Consequently, at large aperture angles, the aberrations become so large that the lens system is no longer useful for high-resolution imaging. At small aperture angles, on the other hand, electron--electron interaction in the beam (the so-called Coulomb interaction) defocuses the beam substantially such that high-current, high-throughput image transfer is impossible.
Conventional MOL and VAL lenses, (as shown, for example, in U.S. Pat. No. 5,466,904,) exhibit problems with astigmatism and field curvature when projecting an off-axis mask subfield. Also, substantial aberrations are caused by the beam deflectors when projecting off-axis subfields.